Several alternative methods are known for machining a workpiece cylindrical surface, such as the main journal on a vehicle crank shaft, from a rough to a finished diameter. In some methods, such as plunge turning or turn broaching, the workpiece is spun about its axis, and the tool is moved into the spinning surface. When it is not practical or desirable to spin the workpiece, it may be held stationary, and the tool spun and moved into the workpiece instead.
This is what is done in a method of milling known as internal disk milling. A ring shaped cutter body has an internal diameter that is substantially larger than the workpiece surface to be machined. Mounted to the inner surface of the cutter body are a plurality of evenly circumferentially spaced cutter inserts located at the same radius and in the same orientation. The workpiece is moved through the cutter body until the surface to be machined is aligned with the inserts, then held stationary. Then, the cutter is operated by spinning the cutter body with its axis parallel to the axis of the workpiece surface, and initially plunged by moving it toward the workpiece, keeping the two axes parallel. Each insert, therefore, moves in a short cutting arc relative to the workpiece surface as it cuts. The plunging motion is stopped when the finished diameter is reached, although only as much of the workpiece surface as the cutting arc of the inserts overlaps will have been machined off. Finally, the axis of the still spinning cutter body is orbited around the axis of the workpiece until the entire surface of the workpiece is machined down to the finished diameter.
An example of a conventional cutter insert used in an internal disk milling cutter may be seen U.S. Pat, No. 4,488,839 to Wermeister et al. The cutter insert 8 is a constant thickness wafer with flat sides that is mounted to the cutter body so as to present one of its straight cutting edge basically perpendicular to its cutting arc direction. The insert may be turned or indexed so as to present new edges to the workpiece with wear. In some cases, polygonal inserts are used, which have a greater number of usable straight cutting edges. The advantage of orienting a straight cutting edge perpendicular to the direction of its cutting arc is that the insert basically pivots about the line of the cutting edge as it moves through its cutting arc. Therefore, the cutting edge does not change its effective orientation relative to the cut surface, and a suitably flat sided cylindrical machined surface results. The big disadvantage of a straight cutting edge so oriented is that it cuts a chip from the workpiece surface that is as wide as the edge is long, creating a high cutting force and edge wear.
Another basic type of surface milling, known as skiving, reduces edge loading by using a diamond shaped insert with a pair of straight cutting edges disposed in a V. The corner of the V contacts the machined surface first, followed by the side edges of the V, which plows the metal to each side. This creates a continuously thinner chip, with consequently less cutting force. The insert has a planar, flat external surface trailing the cutting edges, but is simply tipped back at a suitable clearance angle so that the flat trailing surface does not drag on the machined piece. However, this conventional insert design works well only when it contacts the machined surface in an orientation that basically does not change during the time that the insert sees the workpiece, as is the case in turn broaching or pull broaching. In internal disk milling, where the insert moves through a cutting arc and continuously changes its orientation relative to the machined surface, such a conventional skiving insert, even if tipped back at a clearance angle, would dig into the machined surface and leave a V-shaped trough, rather than a straight sided, "flat" cylindrical surface. It is not immediately obvious how a skiving type cutter insert could be shaped or oriented so as to work properly on an internal milling disk cutter body, which is why the high edge loading of the straight cutting edge has been tolerated.